A variety of techniques have been used to enhance the recovery of hydrocarbons from subterranean reservoirs in which the hydrocarbons no longer flow by natural forces. Such techniques can include water injection and/or subsequent gas flooding, among others. Water injection can be useful to recover some hydrocarbons, however, only about a third of the hydrocarbons are recovered using this technique. As such, typically water injection procedures are followed by gas flooding procedures. Gas flooding can be performed with a miscible gas, to reduce the viscosity of oil present in the reservoir formation in order to increase the flow of hydrocarbons to a production well. Carbon dioxide, which acts as a solvent to reduce the viscosity of the oil, is one of the most effective, and least expensive, miscible gases.
Gas flooding, however, can be accompanied with a number of drawbacks. One main problem encountered is poor sweep of the reservoir formation. Poor sweep occurs when the gas injected into the reservoir during a gas flooding process flows through the paths of least resistance due to the low viscosity of the gas, thus bypassing significant portions of the formation. When the gas bypasses significant portions of the formation, less oil is contacted with the gas, reducing the likelihood that the gas will reduce the viscosity of the oil. Thus, the gas injected during the gas flooding process is meant to “sweep” the oil toward the production well by lowering the viscosity of the oil. However, when the gas does not contact a large portion of the oil contained in the reservoir formation, a large portion of the oil in the reservoir formation is left behind, producing poor sweep. In addition, due to the low density of the gas, the injected gas can rise to the top of the formation and “override” portions of the formation, leading to early breakthrough of the gas at the production well, leaving less gas within the reservoir formation to contact with the oil, again reducing the likelihood that the gas will reduce the viscosity of oil.
To enhance the gas flooding process effectiveness, it has been suggested that the overall efficiency of a gas flooding process can be improved by including a foaming agent or surfactant to generate a foam in the formation. A foam can generate an apparent viscosity of about 100 to about 1,000 times that of the injected gas, therefore, the foam can inhibit the flow of the gas into that portion of the reservoir formation that has previously been swept. In other words, the foam can serve to block the volumes of the reservoir formation through which the gas can short-cut, thereby reducing its tendency to channel through highly permeable fissures, cracks, or strata, and directing it toward previously unswept portions of the reservoir formation. As such, the foam can force the gas to drive the recoverable hydrocarbons from the less depleted portions of the reservoir toward the production well.
The surfactants used in gas flooding processes, however, have suffered from a number of drawbacks. For example, traditional surfactants, such as ethoxy-sulfates, tend to create unstable foams in the reservoir formation. An unstable foam can break and/or dissolve in the reservoir formation, allowing the gas from the gas flooding process to flow into the paths of least resistance, leading to early breakthrough and poor sweep, as discussed herein.
Another problem encountered by prior art surfactants has been the selection of anionic surfactants that have a high affinity to formation rock within the reservoir, for example, carbonate. Surfactants with a high affinity to formation rock can adsorb into the formation rock, leading to surfactant loss. Without the surfactant present, there is less likelihood of forming foam within the reservoir, also leading to early breakthrough and poor sweep, as discussed herein.
Although nonionic surfactants have been used in the prior art to resolve the adsorption problem, the nonionic surfactants chosen have been good emulsifiers of water and oil. As appreciated by one skilled in the art, water flooding is performed prior to the gas flooding process. As such, the reservoir formation is often filled with water when the gas flooding process begins. In addition, since water and oil are present in the reservoir formation, it follows that a mixture of water and oil exit the reservoir formation through a production well. Since water and oil are present in the reservoir formation, a surfactant that creates an emulsion of the oil and water exiting the reservoir formation is undesirable since the oil and water must then be separated to recover the oil, leading to increased production costs.